In the photographic manufacturing industry, it has long been known that the molecular structure of coatings using various natural or synthetic water-permeable hydrophylic polymers, such as gelatin, as vehicles or binders is very complex and dependent on the conditions used to dry the coatings. During setting, the coating liquids are cooled so that the coating turns into a semirigid solid, sometimes termed a gel. Gelation of the coating results from the formation of weak physical cross-links, known as van der Waals cross-links, between the gelatin molecules. To obtain the desired physical and photographic properties, these gel cross-links must not be allowed to be destroyed by melting during the subsequent drying process. Such weak cross-links are easily destroyed by raising the temperature during drying. However, when the coating is properly dried, these weak cross-links are still maintained in the material. It is believed that these weak physical cross-links contribute to the physical properties of the material being produced.
A second important structural parameter of the dried gelatin coating is the glass transition temperature (Tg). Below this temperature, the gelatin molecules are quite rigid and subject to very little thermal agitation. The material is said to be in a "glassy" state. Above this temperature, some of the rigidity of the molecules is lost as the molecules become thermally agitated.
Both the temperature at which the physical cross-links are destroyed and the glass transition temperature are highly dependent on the moisture content. Gelatin is a moderately hygroscopic material. Coatings of gelatin which are in equilibrium with an ambient environment may contain as much as 15 percent water. It is known that the gel melting point, Tm, and the glass transition temperature, Tg, at moderately high moisture content are closely related variables. The molecular agitation above the glass transition temperature would be expected to destroy a portion of the weak physical cross-links, and thus degrade the desired molecular structure of the coating which had been produced by drying the coating at low temperature. While investigating the relationship between gel melting point and glass transition temperature at relatively high temperature and moisture content, it has been found that the chemical hardening reaction could be accelerated to a much greater extent than was thought possible based on previous studies of the effect of storage temperature on the rate of the afterhardening reaction. Furthermore, by choosing the proper combination of temperature and moisture content, acceptable gel structure can be maintained.
The polymeric vehicles or binders are customarily cross linked (hardened) by various organic and inorganic compounds such as those described by T. H. James, The Theory of the Photographic Process, which are often termed hardeners. Hardeners are used to control the amount of swelling which occurs in the layers of the photographic material when it is processed in one or several solutions in order to develop the photographic image from the latent image. In some development processes the binders would dissolve in the processing solutions if they were not hardened before processing. Usually hardening of the binders is accomplished by adding the hardeners to one or more of the liquid photographic emulsions or other layers before they are coated onto the support. The cross-linking reaction starts as the coating is being dried and continues for a long period, often months, after coating and drying. Such hardening after drying and over the long term is often referred to as afterhardening. The rate of the afterhardening reaction is dependent on the temperature and moisture content of the material. Often the hardening reaction is not complete when the photographic material is developed after exposure. More importantly, the photographic response of the material is dependent on the degree to which the hardening reaction has progressed at the time the material is processed. For example, the amount of developer which permeates into the coating when the material is immersed in developer solution is dependent on the degree of hardening which has occurred at this time. This, in turn, influences the rate and extent of the reaction between the silver halide crystals and the developing agents. The problems derived from variations in the extent of afterhardening are especially troublesome for testing and certifying the photographic response of the material during manufacture. Often the dried coated materials are held for some months before final testing and certification and release for customer use, to allow a meaningful certification of the photographic response.
Numerous attempts to accelerate the rate of the afterhardening reaction have been made in which more reactive hardening agents are used. While partially successful, highly reactive hardening agents are very difficult to handle during preparation of the liquid emulsion and coating of the prepared emulsion onto the support. Stagnant areas in the emulsion delivery lines and hopper cavities are very prone to become filled with semisolid hardened binder material which is impossible to flush out without dismantling the delivery lines and hoppers. Furthermore, such hardened binder material may become dislodged as slugs or particles during the coating operation and lead to defects in the coated product.
It has been proposed, for example in Japanese laid open to public inspection Patent Applications (Kokai) 62-81636 and 62-81637, to accelerate the hardening reaction by storing the photographic material at a higher temperature. In these laid open patent applications, the coated and dried photographic material is described as being heated to 40.degree. C. before winding at the end of the machine and the wound rolls are held at this temperature during storage before slitting the rolls to widths used by customers. The time required to complete the hardening reaction was reduced to a few days by this process, but it has been found that the degree of hardening obtained is highly variable unless the temperature throughout the length and width of the rolls are very uniform. Such uniformity is very difficult to achieve in practice. Furthermore, hardening could not be further accelerated by increasing the temperature above 40.degree. C. since to do so would physically damage the coatings in the wound roll.
It is to the solution of this problem of slow and highly variable afterhardening that this invention is directed.